Platinum has long been known as a nearly ideal material for fabricating structures that are in contact with molten glass. Not only is platinum resistant to corrosion from the molten glass, but it also resists oxidation at the high temperatures involved in melting glass. A drawback to platinum, however, is that its strength at high temperatures is not as great as would be desired, and prolonged exposure to high temperatures leads to a degradation of its strength. Providing greater thickness to offset the shortcomings in strength is prohibited by the very high cost of platinum. Higher service temperatures have been made feasible by the use of platinum alloys such as platinum/rhodium, but these alloys do not solve the problem of strength reduction over time. Furthermore, rhodium is even more costly than platinum.
When platinum or a platinum alloy is maintained at elevated temperatures for an extended period of time, loss of strength occurs due to the process of recrystallization. Growth of grains during recrystallization significantly reduces strength because slippage (or "creep") can occur more readily at the boundary between large grains, resulting in failure of the piece. Growth of very large grains sometimes occurs so that the grain boundaries span the entire thickness of the piece, thereby creating a major weakness in the piece.
It was previously found that the high temperature service life of platinum could be increased by means of a dispersed phase of stable refractory oxides within the platinum. Oxides typically employed as the dispersed phase include zirconia (ZrO.sub.2), thoria (ThO.sub.2) and yttria (Y.sub.2 O.sub.3). The presence of the dispersed oxide at the grain boundaries of the platinum inhibits grain growth. This in turn increases the resistance to creep. Oxide dispersion strengthening also serves to improve corrosion resistance of platinum from attack by other metals such as iron or molybdenum. Reference to oxide dispersion strengthened platinum and methods for its manufacture may be found in U.S. Pat. Nos. 3,139,682 (Grant), 3,709,667 (Selman et al.) and 4,252,558 (Touboul et al.). Oxide dispersion strengthened platinum is made by first preparing an alloy with a major portion of platinum (or platinum/rhodium alloy) and a minor portion of metal or metal oxide to be dispersed. If the minor portion is not already oxidized, the alloy is subjected to a treatment that oxidizes the dispersed metal to its oxide within the body of the alloy. The material is then subjected to cold or hot working (rolling, drawing, or forging) so as to impart the desired grain characteristics.
Unfortunately, the manufacturing techniques for oxide dispersion strengthened platinum limit the size of article that can be fabricated from a single piece of the material. In general, it is desirable to avoid joints in equipment for contact with molten glass, so fabrication from the minimum number of pieces is preferred. Welding oxide dispersion strengthened platinum is precluded because heating an area of the metal to its melting point during welding causes that area to lose the dispersion effect, thereby creating an area of weakness at the weld. Hammer welding (impact bonding at elevated temperatures below melting) has been employed to close seams without destroying the dispersion strengthening, but using seams extensively to fabricate a structure is undesirable because of the potential for failure at the seams. A structure requiring a relatively large wall thickness also prohibits attaining the full extent of strengthening by working the metal since the thickness requirement limits the extent of working.